Additives for Cetane Improvement in Middle Distillate Fuels

ABSTRACT

The cetane number of middle distillate fuels may be increased using an additive composition including a polymer that may be a homopolymer or copolymer of olefins, and the like, where the polymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000. The additive composition also includes a free radical initiator component, which may be an alkyl nitrate such as 2-ethylhexylnitrate (2-EHN), and/or a peroxide, such as t-butyl peroxide. In one non-limiting embodiment the amount of polymer in the additive composition is greater than the free radical initiator component. A solvent is also present, which the solvent may include alcohol, an alkyl substituted phenol and/or a heavy aromatic distillate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application from U.S. Ser. No. 12/128,918 filed May 29, 2008, which is incorporated herein by reference, and which in turn claims the benefit of U.S. Provisional Patent Application No. 60/940,914 filed May 30, 2007.

TECHNICAL FIELD

The present invention relates to additives for middle distillate fuels, such as diesel, and more particularly relates, in one embodiment to methods for improving the cetane number of middle distillate fuels using chemical additives.

TECHNICAL BACKGROUND

Cetane number is a measurement of the combustion quality of middle distillate fuels during compression ignition. Middle distillate fuels include heating oil, diesel, kerosene, jet fuel and the like. It is a significant expression of middle distillate fuel quality among a number of other measurements that determine overall middle distillate fuel quality. The cetane number is a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Generally, diesel engines run well with a cetane number from 40 to 55. Fuels with higher cetane number which have shorter ignition delays provide more time for the fuel combustion process to be completed. Hence, higher speed diesels operate more effectively with higher cetane number fuels. There is no performance or emission advantage when the cetane number is raised past approximately 55; after this point, the fuel's performance hits a plateau.

Known cetane improvers include nitrates, peroxides, nitrites, azo compounds and the like and combinations thereof. Alkyl nitrates, especially 2-ethylene hexyl nitrate (2-EHN), may be the most cost effective known additives to improve the cetane number. However, most of these cetane improvers are thermally sensitive, that is, they can undergo self accelerated decomposition under thermal (elevated temperature) conditions. Organic peroxides typically have self accelerated decomposition temperature (SADT) less than 80° C. Some other cetane improvers such as certain nitrite and azo compounds are even shock sensitive, which may pose a dangerous fire and explosive hazard in the field. Even 2-EHN, considered one the safest of the cetane improvers, still has a SADT of about 100° C., which requires special storage tank and engineering control.

Thus, it would be desirable if other additives could be developed to improve the cetane number of middle distillate fuels while be able to reduce or eliminate the amount of common cetane improvers. If the NOx emissions were also reduced this would additionally be beneficial.

SUMMARY

There are provided, in one non-limiting form, additive compositions for increasing the cetane number of middle distillate fuels which composition includes a polymer which includes a homopolymer and/or random or block copolymer of olefins, such as polyisobutylene (FIB), polypropylene (PP), or a random or block polyolefin copolymer and the like. These homopolymers or random copolymers may have a weight average molecular weight (M_(w)) ranging from about 200,000 to about 5,000,000 (0.2-5 MM); alternatively about 900,000 to about 2,600,000 (0.9-2.6 MM). The additive compositions of these polymeric materials also include a free radical initiator component, for instance an alkyl nitrate, such as 2-ethylhexylnitrate (2-EHN), and/or a peroxide, such as an organic peroxides or hydrogen peroxide. The additive compositions also include an alcohol as a solvent and optionally other solvents such as a phenol and a heavy aromatic distillate.

There are further provided in another non-restrictive version middle distillate fuels, such as diesel fuels, heating oil, jet fuels, or kerosene, having improved cetane numbers that contains an amount of an additive composition effective to increase the cetane number. The additive composition includes a polymer that may include a homopolymer of an olefin and/or a random/block polyolefin copolymer and the like, where the homopolymer or random/block copolymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000, alternatively from about 900,000 to about 2,600,000, and a free radical initiator component such as an alkyl nitrate and/or a peroxide, and also contains a solvent such as those mentioned previously, particularly an alcohol.

Also provided in another non-limiting embodiment are methods for improving the cetane number of a middle distillate fuel by adding to the fuel an effective amount of an additive composition that includes a polymer component or combination of polymers that may be a polyolefin homopolymer or a random or block polyolefin copolymer and mixtures thereof, where the homopolymer or random copolymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000. The polymer may be linear, branched or crosslinked. Again, the additive composition additionally contains a solvent such as an alcohol and also includes a free radical initiator component which may be an alkyl nitrate and/or a peroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bar graph of the cetane number for four different middle distillate fuels with no additive, 1000 ppm 2-ethyl hexyl nitrate (2-EHN) and 500 ppm 2-EHN and 500 ppm polymer synergist;

FIG. 2 is a bar graph of the cetane number for CARB middle distillate fuel with no additive, 500 ppm polymer synergist, 500 ppm 2-EHN and 500 ppm 2-EHN plus 500 ppm polymer synergist;

FIG. 3 is a bar graph comparing the cetane numbers from IQT and ASTM D613 methods for Midwestern ULSD fuel for the cases where no cetane improvers were added, where 500 ppm 2-EHN and 500 ppm polymer are used, and where 1000 ppm 2-EHN was added; and

FIG. 4 is a bar graph comparing the cetane numbers from IQT and ASTM D613 methods for CARB Diesel fuel for the cases where no cetane improvers were added, where 500 ppm 2-EHN and 500 ppm polymer are used, and where 1000 ppm 2-EHN was added.

DETAILED DESCRIPTION

The methods and compositions herein relate to raising the cetane number of hydrocarbon fuels and in some cases also reducing the amount of NOx exhaust emissions resulting from the combustion of these fuels in compression ignition engines such as internal combustion engines. In particular, the additives improve (increase) the cetane number of middle distillate fuels. More specifically, the methods and compositions herein concern a fuel additive formulation that includes a relatively high molecular weight polymer and a free radical initiator along with a suitable organic solvent. Suitable polymers are homopolymers or copolymers including, but not necessarily limited to, polyisobutylene, polypropylene, poly alpha-olefin copolymer and the like. The polymers may be linear or branched, and may optionally be crosslinked. The copolymers may be random or block copolymers, or combinations thereof; that is they may have regions which are blocks and different regions which are random. In one non-limiting embodiment the homopolymer or random copolymer has a weight average molecular weight (M_(w)) ranging from about 900,000 independently to about 2,600,000; alternatively, the M_(w) ranges from about 200,000 independently to about 5,000,000. By “independently” throughout the application herein is meant that any lower threshold for a parameter may be combined with any upper threshold. In one non-limiting embodiment, the polymer presence lowers NOx and in many embodiments while also increasing the cetane number.

The additive composition herein also contains a free radical initiator component that may be an alkyl nitrate and/or a peroxide. Suitable alkyl nitrates include, but are not necessarily limited to, 2-ethylhexyl nitrate (2-EHN), CH₃(CH₂)₃CH(C₂H₅)CH₂ONO₂, iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate and the like. Ether nitrates and fatty acid nitrates may also be useful. In one non-limiting embodiment, the alkyl nitrate may function to primarily lower the NOx emissions, but it has been discovered herein to give a synergistic increase in the cetane number when used together with the homopolymer. It is desirable to minimize the proportions of alkyl nitrate since they are relatively expensive and hazardous storage concerns.

The additive composition may also optionally include a peroxide, in place of or in addition to the alkyl nitrate. Suitable peroxides include, but are not necessarily limited to, hydrogen peroxide, di-tertiary butyl peroxide, and benzoyl peroxide, other organic peroxides, and the like. Further, as noted, some synergism has been found between the homopolymer and the alkyl nitrate and/or peroxide. Known cetane boosters for use in distillate fuels include 2-ethylhexyl nitrate, tertiary butyl peroxide, diethylene glycol methyl ether, cyclohexanol, and mixtures thereof. Conventional, known ignition accelerators include hydrogen peroxide, benzoyl peroxide, di-tert-butyl peroxide, and the like.

Other polymers that may also be useful in the additive compositions herein include, but are not necessarily limited to, isotactic polypropylene (such as ones having a weight average molecular weight (M_(w)) in the range of about 2,600,000 or higher) or higher molecular weight hyperbranched polymer products than those described above. Polymer alone without the 2-EHN may be useful in some non-limiting embodiments.

An organic solvent is also useful in the additive compositions described herein, particular an alcohol. Suitable alcohols include, but are not limited to, linear or branched alcohols having 2 to 18 carbon atoms. The hydroxyl group may be terminal or internal. The alcohol may also be substituted with oxygen, nitrogen or sulfur, for instance in a side chain. In an alternative, non-restrictive embodiment the alcohol may be linear or branched alcohols having 4 independently up to 10 carbon atoms, in another non-limiting embodiment from about 4 independently up to 8 carbon atoms. Specific examples of suitable alcohols include, but are not necessarily limited to, butanol, isobutanol, cyclohexanol, and 2-ethylhexanol and the like and mixtures thereof. In one non-limiting embodiment, the alcohol may be from about 1 to about 60 volume % of the additive composition, and in an alternative non-restrictive version the amount of alcohol may range from about 2 independently to about 15 vol % of the additive composition.

Other useful components, which may serve an integration function, such as to hold the components together and protect the integrity of the additive composition, but which may additionally serve to improve the cetane number, include, but are not necessarily limited to, phenols and, quinine or hydroquinone derivatives. In general such derivatives are alkyl substituted phenols, alkyl substituted quinine, or alkyl substituted hydroquinone, where the alkyl group are C1 to C300 hydrocarbyl group that may contain unsaturated double bonds, or triple bonds, may be linear or branched and may be mono, di, tri, tetra, or penta substituted, and may contain O, N, or S, e.g, tocopherol or ubiquinol. Suitable phenols include, but are not necessarily limited to, 2,6-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2,5-di-tert-butylhydroquinone, polyisobutylene phenol, tocopherol (Vitamin E family) and the like and mixtures thereof. Suitable quinone derivatives include, but are not necessary limited to ubiquinol family and the like or mixtures thereof; Suitable hydroquinone derivatives include, but not necessary limited to, tert-butylhydroquinone, 2,5-di-tert-butylhydroquinone and the like or mixtures thereof. In one non-limiting embodiment, the phenol may be from about 0.001 to about 20 volume % of the additive composition, and in an alternative non-restrictive version the amount of alcohol may range from about 0.1 independently to about 10 vol % of the additive composition. Other suitable solvents include, but are not necessarily limited to, heavy aromatic distillates such as those available from Chevron, kerosene or D-2 diesel and the like and mixtures thereof. In one non-limiting embodiment, these other suitable solvents may be from about 0.001 to about 30 volume % of the additive composition, and in an alternative non-restrictive version the amount of alcohol may range from about 0.1 independently to about 10 vol % of the additive composition. The solvent for the homopolymer and the free radical initiator component may include an alcohol, a phenol derivative and a heavy aromatic distillate—all three types together. Other useful components may also include polymer solvation aids including, but not necessarily limited to, limonene, highly branched aliphatic hydrocarbons, and the like.

It has been discovered that the addition of the combination of additives described herein to a diesel fuel, which additives comprise a major/equal amount of polymer with a equal/minor amount of 2-EHN resulted in an increase of cetane number of the fuel to a level surpassing that would be expected based on the single contribution attributed to each component individually. Thus the combination of alkyl nitrate and polymer constitutes an unexpectedly synergetic combination.

The methods herein relate to additive compositions for distillate fuels, as contrasted with products from resid. In the context herein, the methods and compositions are particularly suitable for middle distillate fuels which include, but are not necessarily limited to diesel fuel, heating oil, kerosene, gasoline, jet fuel, and the like. It will be appreciated that middle distillate fuels include, but are not necessarily limited to, blends of conventional hydrocarbons meant by these terms with oxygenates, e.g. alcohols, such as methanol, ethanol, and other additives or blending components presently used in these distillate fuels, such as MTBE (methyl-tert-butyl ether), or that may be used in the future. In one non-limiting embodiment herein, middle distillate fuels include low sulfur fuels, which are defined as having a sulfur content of 0.2% by weight or less, and in another non-limiting embodiment as having a sulfur content of about 0.0015 wt. % useful middle distillate fuels herein are diesel and kerosene. It is expected that a more conventional diesel fuel (i.e. with an aromatic content of >28%) treated with the additive composition herein will be equivalent in emissions to a Texas Low Emissions Diesel (TxLED) fuel with <10% aromatic content.

Generally, in one non-limiting embodiment herein the composition for improving the cetane numbers of middle distillate fuels is a mixture or blend of the free radical initiator component and at least one of the polymers together with an organic solvent, particularly an alcohol. In another non-restrictive version herein the homopolymer or random copolymer is present in the fuel in the range of about 0.01 independently up to about 20,000 ppm, in another non-restrictive version up to about 250 ppm, in one non limiting embodiment from about 0.5 independently up to about 100 ppm; alternatively from about 1 independently up to about 10 ppm, alternatively up to about 5 ppm. The polymer concentration in the additive composition may be from about 2000 to about 40,000 ppm. The free radical initiator component, such as an alkyl nitrate, particularly 2-EHN, may be present in the fuel in the range of about 50 independently to about 4,000 ppm, in another non-limiting embodiment from about 100 independently to about 500 ppm, alternatively from about 200, independently up to about 400 ppm. These amounts are much lower than the amounts of about 1750 ppm 2-EHN alone used in some prior fuel formulations, but by largely replacing the 2-EHN with a homopolymer, similar NOx reductions may be achieved. In one non-limiting embodiment, the volume ratio of polymer to the free radical initiator component ranges from about 1:100 to about 100:1, and alternatively the volume ratio of polymer to the free radical initiator component ranges from about 1:100 to about 100:1; and in one particularly suitable ratio range, from about 1:1 to about 1.5:1; alternatively from about 1:1 to about 3:1.

It will be appreciated that the methods and compositions herein also encompass middle distillate fuels per se containing the additive compositions described herein, as well as methods of improving the cetane numbers of middle distillate fuels using the additive compositions described herein.

Other, optional components of the middle distillate fuels in non-limiting embodiments may include, but are not necessarily limited to detergents, pour point depressants, additional cetane improvers different from those noted as part of the additive composition, lubricity additives, dehazers, cold operability additives, conductivity additives, biocides, dyes, and mixtures thereof. Particularly useful components may include condensation reaction products of aldehydes and amines which are useful as antioxidants and are effective to lower emissions such as particulate matter (PM) and unburnt hydrocarbon (HC). A specific non-limiting example is the condensation reaction product between formaldehyde and di-n-butylamine. In another non-limiting embodiment, water is explicitly absent from the additive composition.

The invention will be illustrated further with respect to the following non-limiting Examples that are included only to further illuminate the invention and not to restrict it.

Examples 1-18

Tables I through IV are compilations of experimental cetane number test results using the ASTM D613 test methodology in a Midwestern, Eastern, Western and CARB Diesel ultra-low sulfur diesel (ULSD) fuels. All tests were performed at Southwest Research Institute (SwRI) unless otherwise noted. The data show that in these ULSDs, the same level of cetane improvement may be achieved by replacing the relatively more expensive 2-EHN with an additive composition containing 0.2% of an about 2,000,000 Mw PIB homopolymer synergist. The results are plotted in FIG. 1.

TABLE I Cetane Synergist Performance Summary Cetane Engine Test Results via ASTM D613 (Midwestern ULSD) Synergist 2-EHN Cetane Number Ex. Dosage, ppm Dosage, ppm Cetane # Improvement 1 0 0 44.8 — 2 0 1000 48.2 3.4 3 400 600 48.4 3.6 4 500 500 48.3 3.5 5 600 400 48.3 3.5

TABLE II Cetane Synergist Performance Summary Cetane Engine Test Results via ASTM D613 (Eastern ULSD) Synergist 2-EHN Cetane Number Ex. Dosage, ppm Dosage, ppm Cetane # Improvement 6 0 0 47.7 — 7 0 1000 49.6 1.9 8 400 600 48.6 0.9 9 500 500 49.6 1.9 10 600 400 50.4 2.7

TABLE III Cetane Synergist Performance Summary Cetane Engine Test Results via ASTM D613 (Western ULSD) Synergist 2-EHN Cetane Number Ex. Dosage, ppm Dosage, ppm Cetane # Improvement 11 0 0 50.4 — 12 0 1000 51.1 0.7 13 500 500 51.4 1.3 14 600 400 51.1 1.0

TABLE I Cetane Synergist Performance Summary Cetane Engine Test Results via ASTM D613 (CARB Diesel) Synergist 2-EHN Cetane Number Ex. Dosage, ppm Dosage, ppm Cetane # Improvement 15 0 0 51.3 — 16 0 1000 53.8 2.5 17 500 500 53.5 2.2 18 600 400 53.5 2.2

Thus, the cetane numbers of the middle distillate fuels is the same as or greater than the cetane number achieved when in the additive composition the free radical initiator component is present at the same total additive dosage level for both the homopolymer and the free radical initiator component combined, and no homopolymer is present.

Examples 19-26

Examples 19-26 present the results of testing from the combination of diluted polymer synergist and 2-EHN which worked better than either additive used individually. In these comparisons, the solvent is believed to help improve the cetane number. The synergist was again 2,000,000 Mw PIB homopolymer. The fuels used were a ULSD and CARB Diesel. The results of Table VI are graphed in FIG. 2. FIG. 2 illustrates that with total 1000 ppm of either polymer synergist and solvent, 2-EHN and solvent, or polymer synergist and 2-EHN, the latter combination surprisingly gives the best results. The solvent was butanol.

TABLE V Synergistic Effect of 2-EHN and Cetane Synergist Cetane Engine Test Results via ASTM D613 (Valero ULSD) 2-EHN Synergist Ex. Dosage, ppm Dosage, ppm Solvent Cetane # 19 0 0 — 50.4 20 0 500 500 47.1 21 500 0 500 49.8 22 500 500 — 51.4

TABLE VI Synergistic Effect of 2-EHN and Cetane Synergist Cetane Engine Test Results via ASTM D613 (CARB Diesel) 2-EHN Synergist Ex. Dosage, ppm Dosage, ppm Solvent Cetane # 23 0 0 — 51.3 24 0 500 500 52.2 25 500 0 500 52.7 26 500 500 — 53.5

Examples 27-29

During testing, all high molecular weight polymers ranging from about 900,000 to 2,100,000 only showed marginal performance when used alone for NOx reduction. It was surprisingly discovered that when about 2,000,000 Mw PIB polymer was used with 2-EHN that a clear synergistic effect was shown that gave a 6.2% NOx reduction as compared with only 0.1% reduction when only the 1% PIB mixture was used and only a 3.2% reduction when 2-EHN was used alone. The fuel used was a typical ULSD from a Valero refinery. The 1% PIB mixture was 1 vol % about 2,000,000 M_(w) PIB homopolymer, 5 vol % butanol and 94% Chevron heavy aromatic distillates. From previous experiments, adding about 5 to about 10% mineral oil may also help lower the NOx emissions. The results are shown in Table VII.

TABLE VII High Molecular Weight PIB Alone, EHN Alone and PIB + EHN EHN Ex. Treat Rate PIB active Conc. % NOx No. Sample (ppm) (ppm) (ppm) Red. 27 BPC91 (0.2% PIB 2000 4 0 0.1 mixture) 28 2-EHN 500 0 500 3.2 29 BPC121 (1% PIB 2000 3.2 400 6.2 mixture + 2-EHN)

Thus, the middle distillate fuel has reduced NOx emissions as compared to an otherwise identical fuel with an additive composition at the same dosage level where: (a) only the free radical initiator component is substituted for both the free radical initiator component and the homopolymer component and/or (b) only the homopolymer component is substituted for both the free radical initiator component and the homopolymer component, where the free radical initiator component is an alkyl nitrate.

Examples 30-39

While determining cetane number via the ASTM D613 engine testing procedure is the industry standard methodology, it is commonplace, especially in refinery sites, to also use a quicker bench method referred to as IQT (Ignition Quality Test) as a guide for assessing fuel. This method is ASTM D6890 and it calculates a derived cetane number.

IQTs were conducted at a third party lab different from SwRI to see how the IQT data compared to the D613 data. The polymer synergist was the same as for Examples 1-26. The expectation was that the data would be comparable under both test methodologies. Table VIII presents the data in tabular form; the amounts of 2-EHN and the polymer synergist were in ppm. FIG. 3 is a bar graph of the results for Examples 35, 39 and 36 for Midwestern ULSD fuel; FIG. 4 is a bar graph of the results for Examples 30, 33 and 34 for CARB Diesel fuel.

However, the initial results from the two fuels indicated that the D613 data and the IQT data were not the same. The IQT showed a bigger response to using straight 2-EHN than the D613 testing did, for unknown reasons. Further investigation would be needed to determine the exact mechanism involved.

TABLE VIII Ex. Fuel 2-EHN Synergist IQT D613 30 CARB Diesel 0 0 50.42 51.3 31 CARB Diesel 334 666 53.75 53.0 32 CARB Diesel 400 600 53.44 53.5 33 CARB Diesel 500 500 54.48 53.5 34 CARB Diesel 1000 0 56.08 53.8 35 Midwestern ULSD 0 0 46.17 44.8 36 Midwestern ULSD 1000 0 49.53 48.2 37 Midwestern ULSD 334 666 47.59 49.2 38 Midwestern ULSD 400 600 48.00 48.3 39 Midwestern ULSD 500 500 48.28 48.3

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective for reducing the emissions of fuels. However, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, specific combinations of polymers together with certain free radical initiator components, e.g. alkyl nitrates and/or peroxides and organic solvents (e.g. alcohols, phenols and/or heavy aromatic distillates) falling within the claimed parameters, but not specifically identified or tried in a particular composition to improve the emissions of fuels herein, are expected to be within the scope of this invention. Certain compositions under certain conditions may serve to improve cetane numbers and/or lower NOx emissions; and/or without any substantial increase in PM emissions or with substantially unchanged PM emissions. It is anticipated that the compositions described herein may also impart to the engines in which they are used as emissions reducers, greater horsepower, and better fuel economy as a result of less friction, whether they are used in diesel or gasoline engines.

The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. For instance, the additive composition may consist of or consist essentially of the homopolymers and the free radical initiators, including one or more solvent such as an alcohol, a phenol, and or a heavy aromatic distillate, as described in the claims.

The words “comprising” and “comprises” as used throughout the claims is to interpreted “including but not limited to”. 

1. A method for improving the cetane number of a middle distillate fuel comprising adding to the distillate fuel an effective amount of an additive composition for improving the cetane number of the middle distillate fuel, the additive composition comprising: a polymer selected from the group consisting of homopolymers and copolymers of olefins, and mixtures thereof, where the polymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000; a free radical initiator component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof; and an alcohol.
 2. The method of claim 1 where: the amount of the free radical initiator component ranges from about 50 to about 4000 ppm based on the total distillate fuel; the amount of the polymer ranges from about 0.01 to about 20,000 ppm based on the total distillate fuel; and the amount of the alcohol ranges from about 1 to about 60 vol % based on the additive composition.
 3. The method of claim 1 where the alcohol is selected from the group consisting of linear or branched alcohols having 2 to 18 carbon atoms and which may be substituted with oxygen, nitrogen or sulfur.
 4. The method of claim 1 where the additive composition further comprises a component selected from the group consisting of an alkyl substituted phenol, an alkyl substituted quinone and an alkyl substituted hydroquinone antioxidant and mixtures thereof, and a solvent selected from the group consisting of kerosene, a heavy aromatic distillate and D-2 diesel, and mixtures thereof.
 5. The method of claim 1 where the cetane number of the middle distillate fuel is the same as or greater than the cetane number achieved when in the additive composition the free radical initiator component is present at the same total additive dosage level for both the polymer and the free radical initiator component and no polymer is present.
 6. The method of claim 1 where the middle distillate fuel has reduced NOx emissions as compared to an otherwise identical fuel with an additive composition at the same dosage level where: only the free radical initiator component is substituted for both the free radical initiator component and the polymer component or only the polymer component is substituted for both the free radical initiator component and the polymer component, where the free radical initiator component is an alkyl nitrate.
 7. A method for improving the cetane number of a middle distillate fuel comprising adding to the middle distillate fuel an additive composition comprising: from about 0.01 to about 20,000 ppm, based on the total distillate fuel, of a polymer selected from the group consisting of homopolymers and copolymers of olefins and mixtures thereof, where the polymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000; from about 50 to about 4000 ppm, based on the total distillate fuel, of an alkyl nitrate; and from about 1 to about 60 vol %, based on the additive composition, of an alcohol selected from the group consisting of linear or branched alcohols having 2 to 18 carbon atoms and which may be substituted with oxygen, nitrogen or sulfur.
 8. The method of claim 7 where the additive composition further comprises a component selected from the group consisting of an alkyl substituted phenol, an alkyl substituted quinone and an alkyl substituted hydroquinone antioxidant and mixtures thereof, and a solvent selected from the group consisting of kerosene, a heavy aromatic distillate and D-2 diesel, and mixtures thereof.
 9. The method of claim 7 where the cetane number of the middle distillate fuel is the same as or greater than the cetane number achieved when in the additive composition the free radical initiator component is present at the same total additive dosage level for both the polymer and the free radical initiator component and no polymer is present.
 10. An additive composition for improving the cetane number of middle distillate fuels comprising: a polymer selected from the group consisting of homopolymers and copolymers of olefins and mixtures thereof, where the polymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000; a free radical initiator component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof; and an alcohol selected from the group consisting of linear or branched alcohols having 2 to 18 carbon atoms and which may be substituted with oxygen, nitrogen or sulfur.
 11. The additive composition of claim 10 where the volume ratio of polymer to the free radical initiator component ranges from about 100:1 to about 1:100.
 12. The additive composition of claim 10 where the amount of alcohol ranges from about 1 to about 60 vol %.
 13. The additive composition of claim 10 where the free radical initiator component is an alkyl nitrate selected from the group consisting of 2-ethylhexyl nitrate (2-EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate.
 14. The additive composition of claim 10 further comprising a component selected from the group consisting of an alkyl substituted phenol, an alkyl substituted quinone and an alkyl substituted hydroquinone antioxidant, and a solvent selected from the group consisting of kerosene, a heavy aromatic distillate and D-2 diesel, and mixtures thereof.
 15. A middle distillate fuel comprising: a middle distillate fuel selected from the group consisting of diesel fuel, heating oil, jet fuel, and kerosene; and an effective amount of an additive composition for improving the cetane number of the middle distillate fuel comprising: a polymer selected from the group consisting of homopolymers and copolymers of olefins, and mixtures thereof, where the polymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000; a free radical initiator component selected from the group consisting of an alkyl nitrate, a peroxide, and combinations thereof; and an alcohol.
 16. The middle distillate fuel of claim 15 where: the amount of the free radical initiator component ranges from about 50 to about 4000 ppm based on the total distillate fuel; and the amount of the polymer ranges from about 0.01 to about 20,000 ppm based on the total distillate fuel; and the amount of the alcohol ranges from about 1 to about 60 vol % based on the additive composition.
 17. The middle distillate fuel of claim 15 where the free radical initiator component is an alkyl nitrate selected from the group consisting of 2-ethylhexyl nitrate (2-EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate.
 18. The middle distillate fuel of claim 15 where the cetane number of the middle distillate fuel is the same as or greater than the cetane number achieved when in the additive composition the free radical initiator component is present at the same total additive dosage level for both the polymer and the free radical initiator component and no polymer is present.
 19. The middle distillate fuel of claim 15 where the additive composition further comprises a component selected from the group consisting of an alkyl substituted phenol, an alkyl substituted quinone and an alkyl substituted hydroquinone antioxidant and mixtures thereof, and a solvent selected from the group consisting of kerosene, a heavy aromatic distillate and D-2 diesel, and mixtures thereof.
 20. A middle distillate fuel comprising: a middle distillate fuel selected from the group consisting of diesel fuel, heating oil, jet fuel and kerosene; and an effective amount of an additive composition for improving the cetane number of the middle distillate fuel comprising a polymer selected from the group consisting of homopolymers and copolymers of olefins, and mixtures thereof, where the homopolymer has a weight average molecular weight ranging from about 200,000 to about 5,000,000; an alkyl nitrate; and an alcohol where the cetane number of the middle distillate fuel is the same as or greater than the cetane number achieved when in the additive composition the free radical initiator component is present at the same total additive dosage level for both the polymer and the free radical initiator component and no polymer is present.
 21. The middle distillate fuel of claim 20 where: the amount of the alkyl nitrate ranges from about 50 to about 4000 ppm based on the total distillate fuel; the amount of the polymer ranges from about 0.01 to about 20,000 ppm based on the total distillate fuel; and the amount of the alcohol ranges from about 1 to about 60 vol % based on the additive composition, all based on the total distillate fuel.
 22. The middle distillate fuel of claim 20 where the alkyl nitrate is selected from the group consisting of 2-ethylhexyl nitrate (2-EHN), iso-propyl nitrate, iso-amylnitrate, iso-hexylnitrate, cyclohexyl nitrate, dodecyl nitrate, diglycol nitrate and tetraglycol nitrate. 